Method of treating titanium-containing structures

ABSTRACT

A method is disclosed for the removal of metallic iron inclusions from the surfaces of chemical process equipment, particularly electrolytic cells, prepared from titanium. This method involves the treating of the titanium alloy surface with an acid solution to remove metallic iron inclusions from those surfaces of the titanium that are subject to attack by halides.

United States Patent 1191 Du Bois 1 51 Sept. 16, 1975 METHOD OF TREATINGTITANIUM-CONTAINING STRUCTURES [75] Inventor: Donald W. Du B015, CorpusChristi,

Tex.

[73] Assignee: PPG Industries, Incl, Pittsburgh, Pa.

[22] Filed: Jan. 4, 1974 [21] Appl. No.: 430,990

Related US. Application Data [62] Division of Ser. No. 239,991, March31, 1972, Pat.

52 us. c1 148/6.l4 R; 117/49; 252/142; 134/41 1511 1111. c1 C23g 1/02{58] Field of Search 148/614 R; 117/49; 134/3, 134/41; 252/142, 143,144, 145

[56] References Cited UNITED STATES PATENTS 2,827,402 3/1958 Albcrs eta1. 252/145 2,830,536 4/1958 Wood et al. 134/41 2,856,275 10/1958 Otto148/614 R 3,457,103 7/1969 Keller et al. 148/6.14 R 3,817,844 Kendall134/3 Primary E.\'ami r 1erRalph S. Kendall Assistant EX'dmirzer-CharlesR. Wolfe, Jr. Attorney, Agent, or FirmRichard M. Goldman [57] ABSTRACT*9 Claims, N0 Drawings METHOD OF TREATING TITANIUM-CONTAINING STRUCTURESThis is adivision, of application Ser. No. 239,991, filed Mar. 31,"1972, and now US. Pat. No. 3,836,410.

BACKGROUND OF THE INVENTION In the construction of chemical processequipment for use in environments where free halogens are present, suchas electrolytic cells for the production of chlorine by the electrolysisof brines, titanium is frequently used as a material of construction.This is because of the tendency of titanium to form a corrosionresistantfilm under oxidizing conditions. However, such titanium reaction vesselsare particularly subject to attack under conditions of oxygen depletionat crevices such as welds, joints, laps, filletts, and the like. Thistype of corrosion is characterized as crevice corrosion."

attempts to prevent crevice corrosion or to substantially reduce theeffects of it have typically focused on various organic coatings on thetitanium, inorganic coatings on the titanium, and the use of titaniumalloys. One particularly satisfactory titanium alloy is an alloy oftitanium and nickel. Such a titanium alloy and its use in halidesolutions is disclosed in US. Pat. No. 3,469,975 to Bertea et al. forMethod of Handling Crevice Corrosionlnducing Halide Solutions". Thealloy disclosed by Bertea et al. is a titanium alloy containing up toabout 5 per cent nickel, at least about 0.3 per cent cobalt, and about2.0 per cent molybdenum. Further disclosed therein is thecorrosionresisting effect of small additions of cobalt and molybdenum totitanium. While such titanium alloys exhibit marked crevice-corrosionproperties when tested as coupons in heated brine, it has been foundthat after various types of mechanical working and fabricatingoperations, their susceptibility to crevice corrosion suffers a markedincrease.

SUMMARY OF THE INVENTION It has now surprisingly been found that thepresence of unalloyed surface iron, even in amounts as low as 5 partsper million, results in a marked increase in susceptibility to crevicecorrosion of titanium and titanium alloys. So sensitive to the presenceof iron is the alloy, that iron inclusion sufficient to materiallyincrease susceptibility to crevice corrosion will be introduced to thealloy during mechanical working. Such inclusions are normally the resultof various metal working and fabricating processes quite apt to be used.7

It has further been found that the crevice corro sionresistantproperties of such worked and fabricated titanium and titanium alloysmay be maintained substantially undiminished if, subsequent to theworking and fabricating processes, particular care is taken to removemetallic iron inclusions therefrom.

According to this invention, metallic iron inclusions are removed bytreatment of the worked or fabricated surface with an aqueous liquidcomposition containing two acids, one of which is an oxidizing acid andthe second of which is capable of reacting with iron to form solubleiron salts.

Best results are obtained if the treatment of the worked or fabricatedtitanium or titanium alloy surface is continued until the amount of ironinclusions remaining in the surface is low enough that thesusceptibility of the article tocrevice corrosion is substantiallyreduced. Such a concentration of metallic iron inclusions is typicallyshown by the negative response of the surface of the article to standardtests for metallic iron.

DETAILED DESCRIPTION OF THE INVENTION According to this invention,titanium or titanium alloy structures subject to corrosion in oxygendefi cient, halogen-containing environments may be rendered resistant tocrevice corrosion by the removal of metallic iron inclusions from thetitanium.

Titanium structures are preferred for use in the processing of halidesand halogens. Such structures include apparatus for the desalinizationof brines and brackish water, electrolytic cells for the production ofchlorate, chlorine, and other halides, and various other chemicalprocessing equipment. Titanium and its alloys are preferred materials ofconstruction because of their tendency to form a corrosion-resistingfilm under oxidizing conditions. This film protects the underliningtitanium material from further corrosive or oxidative attack. However,titanium structures are attacked at laps, filletts, crevices,compression fittings, and the like. They are also" attacked undergaskets at edges of compression fittings, joints, laminations, and thelike. This phenomena is referred to as crevice corrosion."

According to US. Pat. No. 3,469,975 to Bertea et al. for Method ofHandling Crevice Corrosion-Inducing Halide Solutions," it is known thatsmall amounts of alloying materials significantly increase theresistance of titanium structures to crevice corrosion. Such alloyingmaterials include nickel, cobalt, and molybdenum. In the crevicecorrosionresistant alloys of Bertea et al., the quantity ofnickel'present is from about 1.0 per cent to about 5 per cent; theamount of cobalt present is from about 0.3 percent to about 5 per cent;and the amount of molybdenum present is in the range of about 2 percent. Alloyed iron, up to about 0.1 weight per cent may also be presentin the alloy. Additionally, alloys of titanium with the precious metals,such as 0.2 per cent palladium alloysof titanium, have been found to beremarkedly resistant to crevice corrosion. Typical crevicecorrosion-resistant alloys useful in the practice of this invention alsoinclude titanium alloys containing nickel, cobalt, molybdenum, niobium,aluminum, and tantalum. These alloying elements may be present singly asin Ti-Ni", Ti-Co, and Ti-Mo, or in combinations as in Ti-ALNb-Ta-Mo, andTi-Al-Nb-Mo. In such alloys useful with the process of this invention,titanium is the major constituent, being or or more weight per cent ofthe alloy. Whenever titanium is referred to herein it will be understoodto include alloys having titanium as a-rrlajor constituent. It has beenfound that such materials as described hereinabove are particularlyresistant tocrevice corrosion when tested in the form of coupons wherethe crevices are formed by laboratory compressive means. However, thesesame materials suffer a significantly reduced resistance to crevicecorrosion after fabrication into chemical processing apparatus.

It has now been found that the reduced resistance to crevice corrosionof normally crevice corrosionresistant titanium and titanium alloys iscaused by the presence of any metallic iron inclusions in the alloysufficient to cause pitting. This is generally about 5 parts per millionof unalloyed, metallic iron. Such metallic iron inclusions areintroduced during the manufacturing and fabricating steps. They may beintroduced during the i'olling ,bendih'g forging, forming, and shaping asecond acid capable of forming soluble iron salts.

Preferablyfthe'acid treatment is continueduntil the concentration ofmetallic iron in the alloy surface is reduced below that level at whichcrevice corrosion is initiated. This is usuallybelow about 5 parts permillion on the surface and, for practical purposes, below the" levelbelow which standard wet chemical analytical tests for iron arenegative.

The aqueous liquid composition used in the practice of this inventioncontains, as its chemically active ingredients, two acids. The firstacid is an oxidizing acid. By oxidizing acid is meant an acid thatcontains oxygen and that is capable of reacting with titanium to form aninsoluble oxide surface on the titanium. Nitric acid is the preferredoxidizing acid. Perchloric acid and chromic-acid may also be used. Thesecond'acid is an acid capable of reacting with iron to form solublesalts of iron. Suitable acids include the halo acids; hydrofluoric acid,hydrochloric acid, and hydrobromic acid. Hydrochloric acid is thepreferred halo-acid. Hydrofluoric acid may also be used in the practiceof this invention, although care must be taken to stop the treatment ofthe surface before the hydrofluoric acidbegins to solubilize thetitanium. Hydrobromic acid may also be used in the practice of thisinvention although the higher cost of hydrobromic acid may renderit'less attractive than hydrochloric acid. Suitable acids also includestrong organicacids capable of giving up hydronium ions, such as thetri-halogenated acetic acids, trichloroacetic acid and trifluoroaceticacid. Satisfactory results are also obtained with sulfuric acid.

The liquid composition used in the practice of this invention typicallycontains from about 5 to about 20 volume per cent of the oxidizing acidand, preferably, from about 7 /2 to about 15 volume per cent of thetotal liquid composition. When volume per cents are referred to herein,such volume per cents are based on the volumes'of the original reagentsprior to mixing and do not include effects. When the oxidizing acid isnitric acid, the concentration of nitric acid is preferably from about 7/2 to about 15 volume per cent.

The concentration of the second acid isp'referably from about 10 toabout 40 volume per cent. When the second acid is hydrochloric acid,good results are obtained at from about to about 40 volume per cent ofhydrochloric acid. Best results are obtained at about to about volumeper cent hydrochloric acid. When the second acid is hydrofluoric acid,particularly satisfactory results are yielded in the rangeof from about5 to about 10 volume per cent hydrofluoric acid.

In a preferredexemplification of this invention, nitric acid is theoxygeri-containing' acid, and hydrochloric acid is the second 'acid'.According to this exemplification, liquid composi tions havingparticularly satisfactory iron removal properties are provided in therange of from 'about'5 to about IS-volu'mepercent; nitric acid and fromabout 15 to abouti40 volume per cent hydrochloric acid. The preferredliquid composition .of this' exemplification contains from about 7 toabout 15 volume per cent and preferably about 10 volume per cent nitricacid, and from about 20 to about 40 volume per. cent hydrochloric acid,and preferably about 30 volume per cent hydrochloric acid.

Lower concentrations of the second acid, for example, less than about 10volume pencent hydrochloric acid or lower than about 1 volume per centhydrofluoric acid may be used to remove iron inclusions from titaniummaterials. However, such low concentrations of the second acid result inunnecessary long periods of treatment, for example, inn excess of about30 to about 45 minutes. Similarly, particularly high concentrations ofthe second acid, for example, liquid compositions of 40 volume per centhydrochloric acid and 10 volume per cent nitric acid also require inexcess of 45 minutes at 25C. to 30C. to remove substantially all of theiron. I

When the second acid is hydrofluoric acid, liquid compositionscontaining, for example, .10 volume per cent nitric acid'and in excessof about;;10 volume per cent nitric acid and in excess of about 10volume per cent hydrofluoric acid, result in solubilization of thetitanium.

The temperature of the liquid composition is such as to keep it aliquid; that is, between the freezing temperature and refluxtemperature. Particularly satisfactory results are obtained attemperatures of from about 5C. to about 50C. Temperatures above about50C. do not result in any significantly increased rate of removal ofiron inclusions or in decreased treatment time. Furthermore, suchtemperatures give rise. to problems related with solubilization of thetitanium. Accordingly, there is no incentive to go to temperatures aboveabout 50C., although such temperatures are included within the scope ofthis invention. Temperatures below about 5C. result in a significantlyincreased times for the removal of the iron inclusions and, therefore,no incentive exists to go to temperatures below about 5C. although suchtemperatures are also included within the scope of this invention.

The time of treatment varies from about 5 minutes or less up to about4.5 minutes or even longer. As described hereinabove, the time oftreatment is a function ofthe concentrations and proportions of acids inthe liquid composition and the temperature of the liquid composition.For example, at high temperatures the time necessary to, obtainsubstantially complete removal-of iron'as determined by standardcolorimetric tests is less than the time necessary to obtainthe samedegree of iron removal at lower temperatures. Similarly, in a liquidcomposition containing 10 volume per cent nitric acid and 20 volume percenthydrochloric acid, the time required to obtain substantiallycomplete iron removal is less than the time required to obtain anequivalent degree of iron removalw ith liquidcompositions containingsignificantly. more or; significantly less hydrochloric acid.. J, a

In a further exemplification of thisinvention, reduced treatment timesmay-.be obtained :abrading the titanium alloy surface prior, totreatment with the acid compositions. The titanium alloy surface of theapparatus may be. abradedby anysoff the methods normally used for theabrasivecleaping of metal surfaces,

forexample;a power driven wire wheel may be used to remove-surfaceinclusions of metallic iron. In the case removed by the abrasive willbemoreareadily removed by the liquid composition than the ironinclusions dee;

posited during the manufacturing process. Alternatively, sandpaper orsand blasting may be usedias a suit able abrasive method forthepreliminaryremoval of iron inclusion prior to theuseof the acidcomposition.

While the invention has been described and illustrated with respect tocertain alloys of titanium it is to be understood that the tendency oftitanium articles to suffer crevice corrosion may also be reduced by theuse of the acid composition method of this invention either where .othertitanium alloys or unalloye d titanium itself are used. In order thatthose skilled in the art may more completely, understand the presentinvention and the preferred methods by which'the same may be carriedout, the following specific examples are offered:

EXAMPLES [THROUGH VIII In each of Examples 1 through VIII, a 1 inch by 2inch by 0.008 inch titanium coupon was used for testing. An analysis ofthe titanium coupons prior to testing showed a nickeLco'ntent of 1.42per cent and an iron content of 0.007 weight per cent. Each coupon wasthen contam inated with iron on o'nesurface. This was done by clampingthe coupon in a vise 'on a drill press. A blunt iron rod was insertedin'the drill press bit, the drill press turnedon, and' the iron rodpressed onto the coupon.

Thereafter, each coupon was inserted-into an acid solution at atemperature of 25 to 30C. as described hereinafter. Each couponwasremov'edfrom the acid solution after minutes of immersion for thepurpose of testing the surface for iron content. If the coupon showedthe presence ofsurface iron, it was reimmersed in the acid solution for5 minutes and tested-again. This was continued until the test-forsurfaceiron was negative.

The surfaee irons content was determined according to the method of ASTMA-380-57, paragraph 7, (c)( l), by the presence or absence of a darkbluecolor. .According to this method on aqueous indicator solutioncontaining grams of potassium ferric cyanide (K Fe(CH) and millilitersof concentrated ,(70 weight per cent) nitric acid per liter prepared.Each time a coupon was removedfrorn the acid solution, it wasrin'sedwith distilled water andseyeral drops of indicator solution were appliedto' the surface. Iron was considered present if the indicator turneddark blue on the surface of the titanium coupon within 30 seconds. i

EXAMPLE I A titanium alloy coupon prepared and contaminated withiron asdescribed above was inserted in a 10 volume per cent solution of nitricacid. The coupon was tested every 5 minutes of immersion for thepresence of iron for 30 minutes. After 30 minutes there was stillsufficient iron contamination present on the surface to turn theindicator dark blue.

EXAMPLE II A titanium alloy coupon prepared and contaminated 6 with ironas described above was inserted in a 1 volume per cent solution ofhydrochloricacid. The coupon was removed from the acid after every 5minutes ofimmersion and tested, for the presence of' ir on contaminationas describedhereinabove; After3 O'minutesQt here was still sufficientironcontarnination'on the surface to turn the indicator dark blue withinseconds 'a s described hereinabove.

i XAMPLE III A'titanium alloy coupon prepared and contaminated asdescribed hereinabove was inserted in a solution.

containing 10 volume per cent nitric acid and 10 volume per centhydrochloric acid. The coupon was removed and tested forthepresence ofsurface iron contamination after-every 5 minutesof immersion .asdescribed hereinabove. After. 30 -minutes there was still enough ironcontamination on the surface to turn the.

indicator dark bluewithin. 30 seconds as described hereinabove.

EXAMPLE iv A titanium alloy coupon was prepared and contaminated withiron as described hereinabove. It was inserted in a solution containing10 volume per cent nitric acid and 20 volume per cent hydrochloric acid.The coupon was removed from the acid mixture and tested for the presenceof surface iron contamination after every 5 minutes of immersion asdescribed hereinabove. After 20 minutes; the surface'contamination ofiron was reduced sufficiently that the indicator did not turn bluewithin 30 seconds.

. EXAMPLE V a A titanium alloy coupon prepared and contaminated withiron as describedhereinabove was inserted in a'solution'containing 10volume per cent nitric acid and 30 volume per cent hydrochloric acid.After every 5 minutes of immersion the coupon'was removed from thesolution and tested for thepresence of iron as described hereinabove.After 25 minutes there was insuf ficient iron contamination remaining onthe surface of the coupon to change the indicator to dark blue within 30seconds.

EXAMPLE v1, l

EXAMPLE VII A titanium alloy coupon was prepared and contaminated withiron as described hereinabove. The coupon was inserted in a mixture of10 volume per cent nitric acid and 5 volume per cent hydrofluoric acid.The coupon was removed and tested for surface iron contamination afterevery 5 minutes of immersion as described hereinabove. After 20 minutesof immersion in the acid mixture there was no longer sufficient surfaceiron contamination remaining on the coupon to change the indicator bluewithin 30 seconds.

EXAMPLE IX A titanium alloy coupon was prepared and contaminated withiron as described hereinabove. The coupon was brushed with a wire brushsanding wheel for approximately minutes at normal hand pressure.Thereafter the coupon was washed with distilled water and immersed in asolution of volume per cent nitric acid and 20 volume per centhydrochloric acid. The coupon was removed after every 5 minutes ofimmersion to test for iron contamination as described hereinabove. After5 minutes of immersion there still remained on the surface of the couponsufficient iron contamination to turn the indicator blue within 30seconds. After 10 minutes of total immersion time, there wasinsufficient iron remaining on the surface of the coupon to turn theindicator blue within 30 seconds.

EXAMPLE x A titanium alloy coupon was prepared and contaminated withiron as described hereinabove. The coupon was sanded with normal 120 220mesh fine emery cloth at hand pressure for approximately 5 minutes.Vertical and horizontal strokes were applied randomly. Thereafter thesurface of thecoupon was washed with distilled water and inserted in asolution containing 10 volume per cent nitric acid and 20 volume percent hydrochloric acid. The coupon was removed from the solution afterevery 5 minutes of immersion to test iron contamination as describedhereinabove. After 5 minutes of total immersion and after 10 minutes oftotal immersion sufficient iron contamination remained on the surface ofthe coupon to turn the indicator solution blue within 30 seconds. Afterminutes of total immersion there was insufficient iron contaminationremaining on the surface of the coupon to turn the indicator blue within30 seconds.

EXAMPLE Xl A titanium alloy coupon is prepared and contaminated asdescribed hereinabove. A solution containing 10 per cent nitric acid and20 per centsulfuric acid is prepared. The coupon is inserted inthe'acidsolution. After every 5 minutes of immersion in the solution the couponis removed from the solution, rinsed with distilled water, and testedfor the presence of surface iron contamination. After 30 minutes oftotal immersion there is insufficient surface iron contamination to turnthe indicator blue within 30 seconds.

It is to be understood that although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

l claim:

1. A method of removing metallic iron inclusions introduced duringfabrication from fabricated, scale-free titanium structures intended foruse in crevice corrosion susceptible environments which method comprisesI contacting the titanium structure with an aqueous acid compositioncomprising a first acid capable of oxidiz- 'ing titanium which acid ischosen from the group consisting of nitric acid, chromic acid, andperchloric acid, and a second acid chosen from the group consisting ofHF, 'HCl, l-lBr, HI, H CCI COOH, and CF COOH, capable of forming solubleiron salts; and maintaining the acid composition in contact withtitanium until the surface concentration of iron in the titanium isreduced below five parts per million.

2. The method of claim 1 wherein thetitanium is in an alloy comprisingnickel.

3. The method of, claim 1 wherein the oxidizing acid isHNO I 4. Themethod of claim 3 wherein the acid composition comprises from about 5 toabout 15 volume per cent HNO 5. The method of claim-l wherein the secondacid, capable of forming a soluble iron salt, is HCl.

6. The method of claim 5 wherein the acid composition comprises fromabout 15 to about 40 volume per cent HCl.

7. The method of claim 1 wherein the said. aqueous acid compositioncomprises nitric acid and hydrochloric acid. ""8. The method of claim 7wherein the acid composition comprises from about 5 to about 15 volumeper cent nitric acid and from about 15 to about 40 volume per centhydrochloric acid. I

9. Titanium treated by the method of claim 1.

1. A METHOD OF REMOVING METALLIC IRON INCLUSIONS INTRODUCED DURINGFABRICATION FROM FABRICATED, SCALE-FREE TITANIUM STRUCTURES INTENDED FORUSE IN CREVICE CORROSION SUSCEPTIBLE ENVIRONMENTS WHICH METHOD COMPRISESCONTACTING THE TITANIUM STRUCTURE WITH AN AQUEOUS ACID COMPOSITIONCOMPRISING A FIRST ACID CAPABLE OF OXIDIZING TITANIUM WHICH ACID ISCHOSEN FROM THE GROUP CONSISTING OF NITRIC ACID, CHROMIC ACID, ANDPERCHLORIC ACID, AND A SECOND ACID CHOSEN FROM THE GROUP CONSISTING OFHF, HCI, HBR, HI, H2SO4, CCI3COOH, AND CF3COOH, CAPABLE OF FORMINGSOLUBLE IRON SALTS, AND MAINTAINING THE ACID COMPOSITION IN CONTACT WITHTITANIUM UNTIL THE SURFACE CONCENTRATION OF IRON IN THE TITANIUM ISREDUCED BELOW FIVE PARTS PER MILLION.
 2. The method of claim 1 whereinthe titanium is in an alloy comprising nickel.
 3. The method of claim 1wherein the oxidizing acid is HNO3.
 4. The method of claim 3 wherein theacid composition comprises from about 5 to about 15 volume per centHNO3.
 5. The method of claim 1 wherein the second acid, capable offorming a soluble iron salt, is HCl.
 6. The method of claim 5 whereinthe acid composition comprises from about 15 to about 40 volume per centHCl.
 7. The method of claim 1 wherein the said aqueous acid compositioncomprises nitric acid and hydrochloric acid.
 8. The method of claim 7wherein the acid composition comprises from about 5 to about 15 volumeper cent nitric acid and from about 15 to about 40 volume per centhydrochloric acid.
 9. Titanium treated by the method of claim 1.